Assert that the newsletter doesn't contain relative urls

One-sentence description

The newsletter contains relative urls and thus is linking to the admin

Current behaviour / Reproducing the bug

1. Create a newsletter
2. Enter a url without http(s):// in content
3. Send newsletter
4. Open said url
5. Links to django admin

Expected behaviour

No relative url in the newsletter. Thus checking for non-relative urls by checking if there are no mentions of the django admin in the eventual contents.